Sylas Ultimate is Downright Unfair in Games vs Cho'Gath
Normally I don't complain about certain champions or kits being unfair, but this seems like a situation where it's warranted. Sylas getting health stacks by repeatedly stealing Cho's ultimate seems downright game-breaking. Basically you're giving him a way of getting tons of free max health, so he can build pure damage and still be very tanky.
Think about it, Cho'Gath's health stacking makes sense for Cho'Gath's kit. When you give Sylas endless health stacking, it synergizes too well with his kit, and makes him nearly unstoppable once he has a significant number of stacks. It also lets him instantly kill any enemy champion below a certain amount of health, which is crazy for someone who also has a dash, a shield, and 2 forms of cc.